The postal services of many countries around the world permit and/or require the printing of evidence of postage payment, such as a postal indicium, that includes a two-dimensional barcode. Such indicia are commonly referred to as Digital Postage Marks (DPM). For example, the United States Postal Service has implemented a program known as the Information Based Indicia Program (IBIP) which permits a user to generate a postage indicium for sending a mailpiece (e.g., letter, package, etc.) that includes a human readable portion and a machine readable portion in the form of a two-dimensional barcode, such as, without limitation, a Data Matrix symbol.
As is known, a two-dimensional barcode, such as a Data Matrix symbol, typically consists of a number of data regions having nominally square modules arranged in an array, wherein each module generally represents one bit of data. For a black on white Data Matrix symbol, for instance, a darkened (i.e., filled) module represents a binary “one” and a light (e.g., empty) module represents a binary “zero.” Each darkened module typically consists of multiple printed pixels (e.g., in the case of ink jet printing, multiple drops of ink). For example, a darkened module may consist of 25 pixels arranged in a 4×4 or 5×5 pixel pattern. The data regions in a two-dimensional barcode are usually surrounded by a finder pattern which, in turn, is surrounded by a quiet zone border. In addition, multiple data regions may be separated by an alignment pattern.
FIG. 1 illustrates an exemplary “empty” 40×40 Data Matrix symbol 10. More specifically, the symbol 10 shown in FIG. 1 includes 40 rows and 40 columns of modules 11. The left-most column 12 and the bottom-most row 13 of the symbol 10 form an “L” shaped boundary, often referred to as the finder pattern, which is employed to locate the symbol 10, to determine the physical size and orientation of the symbol 10 and/or to determine whether the symbol 10 was distorted when printed. The top-most row 14 and right-most column 15 of the symbol 10 consist of alternating dark and light modules 11 which are employed to define the cell structure of the symbol 10 and/or to assist in determining the physical size and distortion of the symbol 10. As seen in FIG. 1, the symbol 10 is divided into four data regions 16a-16d by a vertical alignment bar 17 and a horizontal alignment bar 18, also sometimes referred to as a finder pattern. As shown in FIG. 1, each data region 16a-16d is comprised of 18 rows and 18 columns of modules 11. FIG. 2 illustrates the Data Matrix symbol 10 of FIG. 1 with information encoded and stored within the four data regions 16a-16d. A detailed description of the Data Matrix symbology can be found in ISO/IEC International Standard 16022:2000(E) entitled “Information Technology—International Symbology Specification—Data Matrix”.
Because the two-dimensional barcodes included in DPMs represent value, and are thus a type of currency, it is tempting to for fraudsters to copy a valid barcode and reuse it on other mailpieces. Even if a system of detection of duplicates is in place at the postal facilities (such as recording the barcodes going through and matching them against a database of all previously recorded barcodes), a number of ways of avoiding detection are possible. For example, the fraudster could send the illegitimate copy of the barcode first and the legitimate barcode afterward, making prosecution practically impossible. Alternatively, the fraudster could send all copies of the barcode at the same time from different locations so that the copies would be processed before the database is updated.
Thus, it is desirable to protect such barcodes against copying. One known way to protect documents and/or images against copying is to use a watermark therein. However, common watermarks provide inadequate protection against two-dimensional barcode copying for at least two reasons. First, most two-dimensional barcodes are a simple graphic printed with a low resolution printer onto an envelope or other paper that has varying and uncontrolled quality. It is therefore difficult to create and adequately hide (i.e., make invisible to the eye) a watermark that is able to withstand the inevitable resulting print quality variation that occurs. Second, not only does the watermark need to be invisible to the eye, but it must also be invisible to the barcode reader (i.e., not effect the reading of the barcode). This requires a higher print quality than is possible with the low resolution printers and paper that are most commonly used. Moreover, a two-dimensional barcode, such as a Data Matrix symbol, is such a simple graphic that the preferred way to produce a copy is not, as with other “natural images,” to do a high quality scan and reprint, but instead is to simply reproduce the barcode (for instance to read the barcode with a barcode reader and regenerate the barcode). Thus, copy detection methods, such as watermark copy detection methods, based on the measure of entropy change during print and scan are inadequate to protect against many fraud efforts.